As related in U.S. Pat. No. 4,933,030 by the same applicant as the present application, a low temperature glass was developed which overcame many of the limitations of the then-existing glass compositions used in Ag/glass die-attach industry. The teachings of the '030 patent are incorporated herein by reference. "Ag/glass" refers to loading a glass with an Ag powder and a suitable organic to make a "paste" which, when applied under an integrated circuit device and correctly processed, will create the required electrical, mechanical, and thermal properties of the resultant assembly. To summarize, the technical advances accomplished by the invention described in U.S. Pat. No. 4,933,030 are:
1. Reduction of the high processing temperatures required to generate adequate adhesion of a die to an integrated circuit package. The performance and yield of high density integrated circuit devices are impaired by high (generally&gt;400.degree. C.) processing temperatures. The capability of attaching these temperature-sensitive devices at lower temperatures (e.g. less than 350.degree. C.) was shown to be very beneficial to the user.
2. Increased reliability and decreased processing of the ceramic packages which house the temperature-sensitive integrated circuit devices. Performing the die-attach process at a temperature less than 350.degree. C. eliminates or greatly reduces the oxidation of the nickel plating under the Au plating as compared to processing at 400.degree.-450.degree. C., the required temperature for prior art lead-borate based Ag/glass system. When processed at 400.degree.-450.degree. C., an additional forming gas process step is generally required to reduce the nickel oxide and achieve adequate wetting of the AuSn pre-forms used to hermetically seal the device in its ceramic package. This improved hermetic yield and the elimination of a costly process step was made possible by the introduction of the product described in U.S. Pat. No. 4,933,030.
3. Elimination of a resin material as part of the organics in a Ag/glass. Prior to the product described in U.S. Pat. No. 4,933,030, Ag/glass compositions always employed the use of a resin to give the paste the proper rheology for applying the Ag/glass material with an automated dispenser, as well as the suspension power and stability of the paste. These resins, typically acrylic in nature, had two major disadvantages when present in any significant amount in an Ag/glass: (1) They were known to retain moisture which contributes to the "residual" moisture sealed in the package. This residual moisture has been shown to significantly and adversely impact the reliability of the final hermetically sealed device; and (2) The resins all have relatively high temperature burn-out properties, typically greater than 300.degree. C. The evolution of the burn-out gases requires a controlled ramp in temperature during the die-attach process to prevent lifting the die, separating it from the Ag/glass adhesive, and causing catastrophic adhesion failures. Thus, the invention described in U.S. Pat. No. 4,933,030 eliminated the resin, thereby reducing the moisture levels, as well as making possible much faster processing during the die-attach cycle.
The invention described in U.S. Pat. No. 4,933,030 did, in fact, overcome many obstacles of the then-existing art, namely, increasing the device and package reliability and significantly reducing the assembly costs by eliminating the separate drying and forming gas clean-up cycle. However, it did leave room for improvement. Although an important advance in other respects, the Tl.sub.2 O.sub.3 /V.sub.2 O.sub.5 /P.sub.2 O.sub.5 glass described in U.S. Pat. No. 4,933,030 did not have the thermal cycling resistance of the established Pb-Borate glass system. Finished devices that utilize these glasses are often subjected to long-term thermal cycling. When parts were cycled from -65.degree. to 150.degree. C. (Mil Std 883, Condition C) for 1000 cycles, the adhesion would substantially decrease in value as a result of the thermal cycling stress. Although the resultant adhesion values would pass Mil Std 883 requirements, they were significantly lower than the values generated by the higher temperature Ag/glass systems.
Various attempts have been made in the prior art to develop glass compositions that are both capable of processing at low temperature (Glass transition temperature, Tg, less than 250.degree. C.) and that have high thermal stress resistance, especially when they are incorporated in Ag/glass compositions. Unfortunately, and generally speaking, as the Tg of a glass is reduced, the thermal expansion is increased making it very difficult to achieve a low temperature Ag/glass with high thermal stress resistance because of the large mismatch of expansion between the silicon semiconductor device (.apprxeq.3 ppm/C) and the low temperature glass which inherently has a very high expansion (15-25 ppm/C). The glass described in U.S. Pat. No. 4,933,030 offers the lowest temperature processing capabilities (processing as low as 300.degree.-325.degree. C.), but does show adhesion degradation of greater than 50% when subjected to the 1000 cycles, condition C of Mil Std 883 (-65.degree. C. to 150.degree. C.).
On the other hand, JMI's Pb-Borate Ag/glass described in U.S. Pat. No. 4,401,767 shows minimum degradation when subjected to the same thermal cycling test, but has a high Tg of .apprxeq.325.degree. C. which necessitates processing the Ag/glass at temperatures greater than 400.degree. C. Thus, it is apparent that there exists a large need in the art for a glass, paste, and method of use which overcomes the above-described problems and gives both a low processing temperature and improved properties, most especially the thermal stress resistance of the existing low temperature systems. The present invention describes a novel approach in the glass design and method of forming that will accomplish these goals.
The prior art (including U.S. Pat. No. 4,933,030) historically speaks of the need to utilize bonds having a glass structure that is substantially non-crystalline, since crystallization of a glass during the processing increases the viscosity and impedes the wetting of the glass to an underlying substrate. For this reason, all the present Ag/glass compositions in the marketplace utilize essentially vitreous glasses, e.g. JMI's Pb-Borate glass described in U.S. Pat. No. 4,401,767; their PbO-V.sub.2 O.sub.5 -Phosphate glass described in U.S. Pat. No. 4,996,171; QMI's Pb-Borate glass described in U.S. Pat. Nos. 4,761,224 and 4,636,254; VLSI's PbO-V.sub.2 O.sub.5 glasses described in U.S. Pat. Nos. 4,743,302 and 5,013,360 and their Ag.sub.2 O-P.sub.2 O.sub.5 glass described in U.S. Pat. No. 4,997,718; and National Starch & Chemical's Ag.sub.2 O-V.sub.2 O.sub.5 -TeO.sub.2 -PbO glasses described in U.S. Pat. No. 4,945,071. The present invention discloses a novel discovery of a low temperature crystallizing glass with superior properties, such as thermal stress resistance, compared to an essentially vitreous glass of a similar Tg. The crystallizing glass of the present invention exhibits a low Tg on early (or low temperative) crystallization (Tc) a low temperature remelt of these crystals (Tr) and a significant crystallization during the cool down of the glass as will be more fully explained later.
Friessen et al in U.S. Pat. No. 4,945,071 describes a TeO.sub.2 /V.sub.2 O.sub.5 /Ag.sub.2 O/PbO system which is essentially vitreous with a glass transition temperature of about 260.degree. C. The crux of the Friessen invention was to keep any crystallization from occuring. Friessen glasses were designed to have no crystal peak temperature or a crystallization temperature that is beyond the processing temperature of the Ag/glasses he described. Particularly key to keeping the glass essentially vitreous (no crystal peak or a high temperature crystal peak) was the addition of PbO as described in Column 5. Also, as will be discussed later in more detail, this oxide state of Pb is not the preferred state for the present invention.
Aside from an entirely different composition, the present invention has several significant distinctions from the above described art. Most significant is that the new glass system of the present invention is based on the fact that a very low temperature crystallization is promoted, rather than prevented along with a very low temperature remelt of these crystals. Furthermore, the crystals remelting create a much lower viscosity glass at a lower temperature than the Friessen glasses and, furthermore, allow processing of semiconductors at substantially lower temperatures (.apprxeq.100.degree. C. lower) while still creating the necessary adhesion along with the associated advantages heretofore described. Unlike the Friessen glasses, the glasses of the present invention end up largely crystallized in the final processed form when cooled, which is considered essential for low temperature glasses to exhibit the desired properties (e.g. thermal cycle endurance) as will be described in detail later.
Likewise, other glass compositions do not teach or suggest the glasses and properties of the present invention. For example, Dumesnil and Finkelstein in U.S. Pat. No. 4,997,718 describe a high Ag.sub.2 O glass composition, including mostly P.sub.2 O.sub.5 and B.sub.2 O.sub.3, as the balance of the composition. These glasses are essentially vitreous, water soluble with high expansion, and fairly low Tg, about 250.degree. C.
Chvatal, in U.S. Pat. Nos. 3,798,114 and 3,853,568, describes high Ag.sub.2 O low temperature glasses that are essentially vitreous, some of which contain combinations of Ag.sub.2 O,V.sub.2 O.sub.5 and TeO.sub.2. Chvatal teaches the use of AgNO.sub.3 as a required batch material for Ag.sub.2 O. The Chvatal patents do not teach the effectiveness of these glass compositions in a Ag/glass die-attach paste, nor do they teach a partially crystalline finished structure or the potential benefits thereof.
Akhtar in U.S. Pat. No. 5,013,697 and Dumesnil and Finkelstein in U.S. Pat. No. 4,743,302 describe sealing glasses, comprising the PbO/V.sub.2 O.sub.5 binary with a combination of other oxides, and low expansion ceramic fillers to produce a series of low melting vitreous sealing glasses.
The present invention is based on the discovery that glasses can be designed to crystallize at low temperature with an accompanying low temperature remelt of that crystal. The resulting glasses, when incorporated in Ag/glasses, will bond at a lower temperature like the Tl.sub.2 O.sub.3 /V.sub.2 O.sub.5 /P.sub.2 O.sub.5 glasses defined in the U.S. Pat. No. 4,933,030, but with marked improvement in properties, especially in thermal stress resistance and chemical durability. These new glasses are characterized by low Tg's (about 250.degree. C. or less), low-temperature crystal formation (about 300.degree. C. or less), a low temperature remelt of these crystals (about 350.degree. C. or less) and a crystalline fired structure with excellent stability after processing.
One of the key results attributed to the present invention is the controlled crystallinity of the finished glass so as to provide a fired glass/ceramic when processed as an Ag/glass paste for die-attach. The controlled crystallization of the fired glassy structure greatly contributes to providing the high adhesions of the die-attach and the resistance to degradation when thermal-cycled, as will be described in detail later. It is believed that the in-situ crystallization provides crystal sites that prevent the propagation of fractures occurring at the silicon die/Ag-glass interface as will be discussed later in detail. There is a large mismatch in expansion between silicon (.apprxeq.3 ppm/C) and low temperature glasses, generally 15-25 ppm/C. As noted earlier, as the temperature properties of a glass are decreased, the expansion coefficient increases creating a need for a partially crystallized structure that provides the resistance to thermal cycling degradation.
Heretofore, one of the major problems of low temperature die-attach materials has been the relatively poor thermal stress resistance, compared to the higher (&gt;400.degree. C.) temperature materials. This invention will describe in detail how the newly disclosed glass system and method overcome the deficiency in thermal stress resistance while retaining the low temperature processing. The addition of low thermal expansion oxides, of about 1-25% by weight, to the novel glass composition for purposes of further improving the the final stress-resistance, and for using the combination as a sealing glass or as an insulating material, is also considered part of this invention as will be disclosed later in detail.